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Multi-Characteristic Optimization in Wire Electrical Discharge Machining of Inconel-625 by Using Taguchi-Grey Relational Analysis (GRA) Approach: Optimization of an Existing Component/Product for Better Quality at a Lower Cost

Abstract

Inconel 625 is a nickel-chromium based super-alloy which is mostly used in high end applications due to its excellent chemical and fabrication properties combined with high strength and outstanding corrosion resistance. Wire EDM is most commonly the used process for machining this material and gives high accuracy and precision in the machined parts. In this paper, an attempt is made to undertake an experimental investigation to optimize the input variables like pulse on time, pulse off time, peak current, and servo voltage in Wire EDM for achieving simultaneous optimization of cutting rate, surface roughness, dimensional deviation, and wire wear ratio. Taguchi's L9 orthogonal array with one replication has been used as an experimental design and then analysis of variance (ANOVA) has been applied to determine the significant parameters and their contributions on response variables. Taguchi method has been combined with grey relational analysis for multi-characteristic optimization. Finally, results are validated with the help of confirmation experiments.

Introduction

Today’s manufacturing industries promote the use of modern and sophisticated materials like ceramics, super alloys, composites etc. for their manufacturing base (Singh, 2012). These materials provide greater mechanical strength without increasing the weight of overall component. However, the major problem with these materials the type of machining they require for their processing as they cannot be economically machined by conventional tools available (Rao and Kalyankar, 2014). The material used in this study is a nickel-chromium based super alloy Inconel 625. This material is an excellent choice for sea water applications, aerospace field, chemical processing industries and nuclear energy sector due to its ultimate properties (Yan et al., 2013).

In this research work, the material is machined with the help of WEDM which is presently the most versatile machine tool available for machining of electrically conductive materials. WEDM is a non - convectional machining process used in aerospace, die making industries, etc. due to high accuracy and precision work obtained. This machining process is capable to machine two dimensional and three dimensional intricate shape and design due to very low material removal rate (Tosun et al., 2003). When the suitable voltage is applied, discharge occurs between the wire and work material where a suitable gape is maintained between them. Dielectric like deionized water is applied in the gape which acts as insulation resistance. Due to sparking process temperature reaches about 8300 – 11000 °C. The micro-chips formed in WEDM are carried away by the dielectric fluid and which also cools the workpiece. Wire is fed continuously maintaining the gape of 0.025 to 0.05mm between the wire and work (Sharma et al., 2013). There are many types of wire like brass wire, copper wire and coated wire etc. work as cutting tool in WEDM so that manufacturing of costly tool is avoided (Maher et al., 2015). WEDM is thermo- electric process in which electrical energy is converted into heat by which spark is produced. This spark melts and vaporizes the work piece in the form of small burr which is carried away by the dielectric fluid. In this process, electrode and work piece are separated by small distance, so mechanical stress is negligible. Generally deionized water is applied as a dielectric fluid which helps in producing spark and maintaining gape between work material and wire electrode (Alias et al., 2012). The wire movement is controlled by the Computerized Numerical Control, so that the accuracy obtained is very high. Taper cutting is also possible by the WEDM with the help of CNC controller. It is capable to machine all electrically conductive materials irrespective of their toughness and hardness. WEDM is mostly used in automotive, aerospace, mould, tool and die making, medical, optical, dental, and jewelry industries (Ramakrishnan and Karunamoorthy, 2008). Figure 1 represents the WEDM process.